Check to make sure that all RCAs are properly seated as well as all other connections.
Then you can begin the trial and error process of elimination.
First, you must have a visual on the signal path. Draw a sketch to keep it straight.
Second, you must label all terminations before disconnecting a single wire or cable. You must leave bread crumbs.
Third, you must identify what components in the signal path are shared and not shared by the two speakers that are out.
Fourth, you will reverse RCA cables and/or speaker wires, at either end, one at a time. By determining when the problem switches to another product and when it does not, you will have eliminated potential sources and isolated the true culprit.